Fault current limiting

Abstract

An alternating current system 10 has a primary circuit 11 which forms a primary winding 18 on a core 16 . A secondary winding 24 is connected with a current source 26 or, alternatively, with an impedance 60 . The core 16 is threaded by a superconducting coil 20 having a current source 22. In normal use, current in the coil 20 provides a DC bias level of flux in the core 16 , and the source 26 is varied to maintain substantially constant flux, thereby minimizing losses in the primary circuit 11 . In fault conditions, current in the coil 20 is reduced or removed to increase voltage losses across the coil 18 , thereby limiting fault current. The impedance 60 can also be switched into circuit, creating further current limiting by virtue of the transformer effect of the windings 18, 24.

1. A fault current limiting arrangement, comprising:
 a primary circuit for current to be limited in the event of a fault, 
 a core having a primary winding forming part of the primary circuit; 
 a coil coupled with the core, and a DC current supply for the coil, to provide a bias level of flux in the core; 
 an auxiliary winding coupled with the core, wherein the core comprising first and second arms, the second arm being thicker than the first arm, the primary winding being wound around the first arm, and the auxiliary winding being wound around the second arm; and 
 an auxiliary current source for the auxiliary winding, means to adjust the output of the auxiliary current source in response to changes at the primary winding to apply control to the primary winding by control of the flux within the primary winding, wherein the auxiliary current source is operable to control the auxiliary current to maintain substantially constant flux within the primary winding, during use, except during fault conditions. 
 
   
   
     2. An arrangement according to  claim 1  wherein the coil comprises a superconducting element. 
   
   
     3. An arrangement according to  claim 1  wherein the coil DC current supply is operable, in use, to set the coil current to saturate the core. 
   
   
     4. An arrangement according to  claim 1  wherein the coil DC current supply is operable, in use, to change the coil current in response to a fault condition occurring in the primary circuit. 
   
   
     5. An arrangement according to  claim 4  wherein the coil DC current supply is operable to remove the coil current in response to a fault condition. 
   
   
     6. An arrangement according to  claim 1 , wherein the primary circuit is connected with one phase of a multi-phase electrical system, the auxiliary current source being operable to control the auxiliary current to change the primary winding flux, thereby to provide VAR compensation. 
   
   
     7. An arrangement according to  claim 1 , wherein the auxiliary winding forms an auxiliary circuit, the auxiliary circuit includes an impedance and switches for switching the impedance into the auxiliary circuit during fault conditions. 
   
   
     8. An arrangement according to  claim 1 , wherein in use, the bias level of flux provided by the coil is sufficient to saturate the first arm and is insufficient to saturate the second arm in the presence of current in the primary or auxiliary windings. 
   
   
     9. A method of fault current limiting in which:
 a core is provided having a primary winding forming part of a primary circuit; 
 a coil is coupled with the core, and provided with DC current to provide a bias level of flux in the core;
 an auxiliary winding is coupled with the core, wherein the core comprising first and second arms, the second arm being thicker than the first arm, the primary winding being wound around the first arm, and the auxiliary winding being wound around the second arm; and 
 current is provided for the auxiliary winding and adjusted in response to changes at the primary winding to apply control to the primary winding by control of the flux within the primary winding, wherein the auxiliary winding current is controlled to maintain substantially constant flux within the primary winding, during use, except during fault conditions. 
 
 
   
   
     10. A method according to  claim 9 , wherein the coil comprises a superconducting element. 
   
   
     11. A method according to  claim 9 , wherein the coil current supply sets the coil current to saturate the core. 
   
   
     12. A method according to  claim 9 , wherein the coil current changes in response to a fault condition occurring in the primary circuit. 
   
   
     13. A method according to  claim 9 , wherein the coil current is removed in response to a fault condition. 
   
   
     14. A method according to  claim 9 , wherein the primary circuit is connected with one phase of a multi-phase electrical system, and the auxiliary winding current is used to control the auxiliary current to change the primary winding flux, thereby to provide VAR compensation. 
   
   
     15. A fault current limiting apparatus for a primary circuit comprising:
 a core; 
 a coil; 
 a primary winding; 
 a DC current supply; 
 an auxiliary winding; 
 an auxiliary current source; 
 a first detector; 
 a second detector; 
 at least one switch; and 
 a conductor; 
 said core comprising a first arm, a second arm and two legs so that said legs connect said first arm with said second arm to form a closed loop, said second arm being thicker than said first arm; 
 said primary winding being part of a primary circuit and being coupled with said core and comprising said conductor wrapped around said first arm; 
 said first detector being constructed and arranged to detect changes in at least one of a current in primary winding, a voltage across the primary winding, or flux in the core; 
 said second detector being constructed and arranged to detect a fault condition in the primary winding; 
 said auxiliary winding being coupled with said core and comprising said conductor wrapped around said second arm; 
 said coil being coupled with said core; 
 said DC current supply being connected to said coil to induce magnetic flux in said core such that said induced magnetic flux operates at a DC bias set by current in said coil; 
 said auxiliary current source being connected to said auxiliary winding and said first detector to provide a feedback arrangement which maintains a substantially constant magnetic flux within said core; and 
 said second detector being connected to said DC current supply so that, in the event of a fault, the DC current supply is switched so that the current in coil is no longer maintained and reduced to zero wherein, in response to the onset of fault conditions, said at least one switch disconnects said auxiliary current source from said auxiliary winding to protect the coil.